Derivatives (Credit Derivatives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	$ 88	$ 74
Maximum Amount of Future Payments under Credit Default Swaps	8,909	8,879
Weighted Average Years to Maturity	3 years 8 months 12 days	3 years 7 months 6 days
Aaa/Aa/A [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	45	52
Maximum Amount of Future Payments under Credit Default Swaps	3,430	3,490
Weighted Average Years to Maturity	2 years	2 years 2 months 12 days
Aaa/Aa/A [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	9	10
Maximum Amount of Future Payments under Credit Default Swaps	717	777
Weighted Average Years to Maturity	2 years 6 months 0 days	2 years 8 months 12 days
Aaa/Aa/A [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	36	42
Maximum Amount of Future Payments under Credit Default Swaps	2,713	2,713
Weighted Average Years to Maturity	1 year 9 months 18 days	2 years 1 month 6 days
Baa [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	33	19
Maximum Amount of Future Payments under Credit Default Swaps	5,154	5,064
Weighted Average Years to Maturity	4 years 8 months 12 days	4 years 6 months 0 days
Baa [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	12	8
Maximum Amount of Future Payments under Credit Default Swaps	1,209	1,314
Weighted Average Years to Maturity	3 years 4 months 24 days	3 years 4 months 24 days
Baa [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	21	11
Maximum Amount of Future Payments under Credit Default Swaps	3,945	3,750
Weighted Average Years to Maturity	5 years 1 month 6 days	4 years 10 months 24 days
Ba [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	25	25
Weighted Average Years to Maturity	2 years 4 months 24 days	2 years 8 months 12 days
Ba [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	25	25
Weighted Average Years to Maturity	2 years 4 months 24 days	2 years 8 months 12 days
Ba [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	0	0
Weighted Average Years to Maturity	0 years	0 years
B [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	10	3
Maximum Amount of Future Payments under Credit Default Swaps	300	300
Weighted Average Years to Maturity	5 years 2 months 12 days	4 years 10 months 24 days
B [Member] | Single name credit default swaps (corporate) [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	0	0
Maximum Amount of Future Payments under Credit Default Swaps	0	0
Weighted Average Years to Maturity	0 years	0 years
B [Member] | Credit default swaps referencing indices [Member]
Credit Derivatives [Line Items]
Estimated Fair Value of Credit Default Swaps	10	3
Maximum Amount of Future Payments under Credit Default Swaps	$ 300	$ 300
Weighted Average Years to Maturity	5 years 2 months 12 days	4 years 10 months 24 days